Liquid level sensing system and apparatus

ABSTRACT

First and second pairs of electrodes are partially immersed into liquid on opposite sides of a cleaning screen with the electrodes on the discharge side being immersed to a greater depth than the electrodes on the input side of the screen. The electrodes are connected in a bridge circuit, the output of which is connected in series with the secondary winding of a step-down transformer supplied from a primary alternating current signal source. This series circuit is connected to the grid electrode of a triode which controls an alarm relay. The electrodes in each of the pairs are parallel stainless steel electrodes separated by insulating spacers.

United States Patent 1 1 1 1 55,804 Johnson Aug. 28, 1973 LIQUID LEVELSENSING SYSTEM AND APPARATUS Walter P. Johnson, 510 S. Smith St.,Aurora, lll.

Filed: Oct. 18, 1971 Appl. No.: 190,135

Inventor:

References Cited UNITED STATES PATENTS Erickson Greenwood PrimaryExaririner-'l'homas B. Habecker Attorney-La Valle D. Ptak et al.

[5 7] ABSTRACT First and second pairs of electrodes are partiallyimmersed into liquid on opposite sides of a cleaning screen with theelectrodes on the discharge side being immersed to a greater depth thanthe electrodes on the input side of the screen. The electrodes areconnected in a bridge circuit, the output of which is connected inseries with the secondary winding of a step-down transformer suppliedfrom a primary alternating current signal source. This series circuit isconnected to the grid electrode of a triode which controls an alarmrelay.

The electrodes in each of the pairs are parallel stainless steelelectrodes separated by insulating spacers.

10 Claims, 4 Drawing Figures PATENTEDAU828 ms 3.755504 l2 fi I FIG. I

. W i WATER FLOW LEVEL FIG 2 22 23 20 I7 FLOW LIQUID LEVEL SENSINGSYSTEM AND APPARATUS BACKGROUND OF THE INVENTION In supplying water tothe condensers ofa power plant or to the pumping station of a waterfiltration plant, it generally is necessary to screen solid debris, suchas sticks, leaves and refuse of various types, out of the incomingstream of water to keep such debris out of the narrow passages of thecondensers or pumps. Generally, a continuously rotating cleaning screenis employed for this purpose, and the screen is placed across thechannel between the main reservoir of the water supply and the inletpool to the pumps or condensers. Due to the fact that much of therotating mechanism for the cleaning screen is continuously immersed inthe water, relatively rapid wear of the equipment takes place so thatfrequent and expensive maintanence often is necessary.

In many systems little debris is encountered much of the time, so thatsuch continuous operation of the cleaning screen would not be necessaryif a reliable means for detecting when the screen was becoming cloggedcould be used to initiate operation of the screen only when operationwas needed.

In addition there are occasions when, due to storms, floods iceconditions or the like, an operating screen may become clogged to theextent of substantially diminishing the flow of water through the screenfrom the main reservoir to the water inlet pool on the downstream sideof the screen. If the water is pumped from the pool at a rate fasterthan water passes through the screen, the difference in pressure onopposite sides of the screen can become sufficient to cause a collapseor destruction of the screen. Even if the screen were not destroyed bysuch a difference in pressure, the supply of water in the inlet poolcould become sufficiently low due to screen-clogging that the equipmentprovided with the water could be damaged because of an inadequate watersupply. It is important therefor to be able to accurately detectdifferences in water level on the reservoir or input side of a cleaningscreen and the inlet pool or output side of such a screen wheneverdifferences in water levels on opposite sides of the screen occur forany reason.

A system for detecting differences in liquid level on opposite sides ofa cleaning screen is disclosed in U.S.

Pat. No. 2,659,068 to Erickson and Johnson. The sys-v tem disclosed inthis patent is satisfactory when the absolute level of the water in themain reservoir supplying water through the screen to the inlet pool doesnot undergo fluctuations of more than a relatively few feet (of theorder of 5 or 6 feet) due to seasonal variations or other causes. Whenthe absolute water level of the main reservoir is subject to extensivevariations, so that long electrodes must be used, the system of theabove mentioned Erickson/Johnson patent requires an auxillerycompensating circuit to compensate for the wide variations in impedancewhich occur with variations in the depth of the water supply in thereservoir. Such an auxillery compensating circuit increases the expenseof the system and also increases its complexity.

SUMMARY OF THE INVENTION IT is an object of this invention to provide animproved liquid level sensing system.

It is an additional object of this invention to provide a simple andreliable system for detecting differential liquid levels.

It is a further object of this invention to provide an improvedelectrode structure for a liquid level sensing device.

It is yet another object of this invention to provide a differentialliquid sensing system which provides reliable operation in spite of widefluctuations in the absolute level of the liquid supply.

In a preferred embodiment of this invention, a system for sensingdifferences in the level of liquids at first and second spaced pointsincludes first and second pairs of spaced conductive electrodes, eachlocated for partial immersion into the liquid at the first and secondspaced points, respectively. A resistance is connected between the upperends of one of the electrodes of each of the pairs and the otherelectrodes of each of the pairs are connected together at a common pointto form a bridge circuit. The primary winding of a bridge transformer isconnected between an intermediate point on the resistance and the commonpoint to provide an output from the bridge cicuit on the secondarywinding of the bridge transformer.

The primary winding of afirst step-down transformer is connected to aconventional alternating current source, and has a secondary windingcoupled to the junctions of the resistance with the upper ends of theelectrodes to provide an input to the bridge circuit. The secondarywinding of a second step-down transformer, the primary of which isconnected with the alternating current source, is connected in seriescircuit with the secondary winding of the bridge transformer to controlan indicating device.

In a more specific embodiment of the invention, the second pair ofelectrodes is extended to a greater depth in a liquid than the firstpair to unbalance the bridge for the same liquid level at both points.This unbalance is translated by the secondary winding of the bridgetransformer as a phase opposition bucking signal to the signal obtainedfrom the secondary winding of the second step-down transformer to causea first or normal output condition from the indicating device. When thebridge becomes balanced, due to a drop in the liquid level at the secondpoint relative to the liquid level at the first point, no output isobtained therefrom. The output from the secondary winding of the secondstep-down transformer then is sufficient to operate the indicatingdevice to a second condition to indicate that a predetermineddifferential in liquid level between the first and second points exists.A further unbalancing of the bridge, caused by a further drop in theliquid level at the second point relative to the first point, causes areversal in the phase of the signal in the secondary winding of thebridge transformer to augment the signals causing operation of theindicating device to its second condition. Each of the electrode pairsin a preferred embodiment consist of parallel stainless steelelectrodes.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of apreferred embodiment of the invention;

FIG. 2 illustrates details of the electrode structure and placement ofthe sensing portions of the system shown in FIG. 1; and

FIGS. 3 and 4 show details of the electrode structure of FIGS. 1 and 2.

DETAILED DESCRIPTION In the drawing, the same reference numerals areused throughout the several Figures to designate the same or similarparts.

Referring first to FIG. 1, there is shown an alternating current supplysource which is connected through a suitable switch 11 to a pair ofalternating current supply leads 12 and 13. The source 10 supplies theusual house voltage of from llO to 120 volts, 60 Hertz AC as aconvenient source of power for the system. The alternating currentsource, of course, could be of different voltages and frequencies, butthe one desceibed is convenient since it is practically universallyavailable.

The primary alternating current signal which is present on the leads l2and 13 is supplied to a differential liquid sensing system 15 as theoperating power for the system. The sensing system 15 is utilized inconnection with a water supply system, illustrated as including a stream16 which is confined within a suitable channel or tunnel having a bottomwall 17 and a pair of side walls (not shown). A suitable rotatablecleaning screen 18 is located within the channel between the reservoiror input side shown to the left of the screen in FIG. 1 and the inletpool or output side shown to the right of the screen 18 in FIG. 1. Thescreen 18 is shown as being driven by a drive roller 19, located abovethe surface of the water, with an idler roller located below the surfaceof the water near the bottom of the channel 17. The screen 18 generallyis constructed of metal and may be of a suitable conventionalconfiguration, the details of which are not important to anunderstanding of the liquid level sensing System15.

As is well known, the function of the screen 18 is to remove debris andsolid impurities which are intercepted by it to keep such debris andimpurities from clogging the equipment using the water obtained from theinlet pool to the right of the cleaning screen 18. The screen 18 may becontinuously oprated by continously driving the driving wheel 19; but iflittle or no debris is carried by the flow of water from the reservoirto the left of the screen, such continuous operation results inunnecessary wear.

Whether the screen is intermittently operated or continuously operated,however, it is subject to clogging; so that it is desirable toaccurately detect when the water level on the pool or outlet side of thescreen drops to some predetermined level below the level of the water onthe input or reservoir side of the screen.

Such a drop in the water level on the outlet side of the screen 18 canoccur if the screen becomes sufi'iciently clogged to restrict the flowof water through the screen to an amount less than the rate at which thewater is withdrawn from the inlet pool to the right of the screen.

To permit detection of the differential water levels on the two sides ofthe screen 18, a first pair of elongated electrodes 22 and 23 arepartially immersed on the reservoir side of the screen 18 to a depthsufficient to insure that a substantial portion of the electrodes 22 and23 are always immersed, irrespective of fluctuations in the reservoirwater level which may take place. A sec ond pair of electrodes 25 and 26are partially immersed into the water on the inlet pool side of thescreen 18 and extend from a point well above the normal level 28 of thewater to a point which is a fixed preestablished distance below thebottom ends of the pair of electrodes 22 and 23. This distance isindicated as distance D(illustrated in FIG. 2 which shows the two pairsof electrode assemblies on opposite sides of the cleaning screen 18).

The upper ends of the electrodes 23 and 26 are interconnected in commonto ground. A pair of resistors 29 and 31 are connected between the upperends of the electrodes 22 and 25 at a pair of terminals 33 and 35; sothat the two pairs of electrodes 22, 23 and 25, 26 and the two resistors29 and 31 form a Wheatstone bridge circuit. An alternating currentpotential is supplied to the input terminals 33 and 35 of the bridgecircuit through the secondary winding 36 of a step-down transformer 37,the opposite ends of the primary winding 39 of which are connected tothe leads 12 and 13. The transformer 37 is used to reduce the potentialon the grounded portion of the system to about six or twelve volts. 1

The bridge preferably is constructed with the resistors 29 and 31 ofequal value. The resistances of the other two arms of the bridgeconstitute the resistance between the electrodes 22 and 23 for one ofthe other arms and the resistance between the electrodes 25 and 26 forthe other of the arms. The values of these resistances'are determined bythe conductivity of the water, the contact resistance of the water andelectrodes and the depth of immersion of the respective electrode pairs22, 23 and 25, 26. Output terminals of the bridge constitute a junction40 between the resistors 29 and 31 and the ground terminal to which theupper ends of .the electrodes 23 and 26 are connected.

If the resistors 29 and 31 are equal in value and if the liquid level isthe same on both sides of the screen 18, as indicated by the level 28, agreater resistance exists between the electrodes 22 and 23 than existsbetween the more deeply immersed electrodes 25 and 26. This causes thebridge to be unbalanced by an amount determined by the distance D (FIG.2). Thus, a potential exists between the junction 40 and ground withequal water levels on both sides of the cleaning screen 18. Themagnitude of this potential is determined by the values of theresistances in the arms of the bridge, and the phase of it is determinedby which of the electrode pairs 22, 23 and 25, 26 has the greaterresistance. This output potential is applied to the primary winding 43of a step-up transformer 44, having a high voltage secondary winding 45,one end of which is connected to the grid of a triode vacuum tube 48through a resistor 49. The other end of the winding 45 is connectedthrough the tap of an adjustable potentiometer 50 to the cathode of thetiode 48 to control the grid bias of the triode 48.

The secondary winding 45 of the transformer 44 is connected in a seriescircuit with the secondary winding 54 of a second step-down transformer55 (similar to the transformer 37), the primary winding 56 of which isconnected across the power supply means l2 and 13. With the bridgeinitially unbaianced in the manner described above, the signals on thesecondary winding 45 are in phase opposition to the signals induced inthe secondary winding 54 of the transformer 55. The relative amounts ofthese signals applied across the grid-cathode circuit of the tube 48 areadjusted by adjusting the tap on the potentiometer 59, since one end ofthe secondary winding 54 is connected directly to the cathode of thetube 48; and the other end is connected through the lower portion of thepotentiometer 50, the winding 45, and the resistor 49 to the grid of thetriode 48.

In the absence of any signals induced in the secondary winding 45, thephase and magnitude of the signals applied by the secondary winding 54to the grid of the triode 48 are sufficient to cause the triode 48 to berendered nonconductive. This state of operation corresponds to an alarmstate of operation.

As stated previously, the phase of the signals induced in the secondarywinding 45 of the transformer 44, however, for the unbalanced conditionof the bridge circuit, indicating that the water level 28 is the same onboth sides of the cleaning screen 18, operates to cause an out-of-phasebucking signal to be combined with the signal from the secondary winding54. The magnitude of this out-of-phase signal is sufficient to preventthe signal applied by the secondary winding of the transformer 54 fromrendering the tube 48 nonconductive, and sufficient forward bias isapplied to the grid of the tube 48 to cause it to be normallyconductive. A capacitor 58 is connected across the ends of the winding54 to correct the power factor of the grid circuit of the tube 48 tomaintain the alternating potential in the grid circuit in proper phaserelationship with the potential applied to the plate circuit to maintainthe tube conductive for the state of operating conditions justdescribed.

A relay 60 has an operating winding 61 connected in series with theplate circuit of the tube 48, with one terminal of the winding 61connected to the plate of the triode 48 and the other terminal connectedto the lead 12. The cathode of the triode 48 is connected directly tothe other power supply lead 13, and a capacitor 63 is connected acrossthe winding 61 to keep the armature 64 of the relay 60 from chatteringand to provide additional power factor correction.

With the water level 28 maintained as shown in FIGS. 1 and 2 of thedrawing, sufficient bucking potential is applied by the winding 45 inseries with the control winding 54 to cause the triode 48 to bemaintained conductive. Current then normally flows through the platecircuit and energizes the winding 61 of the relay 60, causing thearmature 64 to be retracted to the position shown in FIG. 1 therebyopening the circuit between a pair of stationary contacts 66 and 67.

When the liquid level on the right-hand or output side of the screen 18drops to a distance D below the level on the input or left-hand side ofthe screen 18, the bridge becomes balanced due to equal resistancesappearing between the electrodes of each electrode pair 22, 23 and 25,26. When this occurs, no potential then is applied across the primarywinding 43 of the step-up transformer 44, and no bucking potential isinduced in the secondary winding 45. Thus, the full potential availablefrom the secondary winding 54, as determined by the step-down ratio ofthe transformer 55, the value of the resistor 49 and the setting of thetap of the potentiometer 50, is applied to the grid of the triode 48.The phase and magnitude of this potential is selected to be sufficientto decrease the plate current of the triode 48 to a level which isinsufficient to maintain the relay 60 energized or operated. This causesthe armature 64 to be released, bridging the contacts 66 and 67. Whenthis occurs, a circuit is completed from a battery 71 through an alarm72 and through an operating motor 73 to drive the drive wheel 19 of thescreen 18 through a suitable mechanical link indicated by the dashedline 74 in FIG. 1.

The alarm 72 may be of any suitable form, such as a light, audiblealarm, or other indication; and the control motor or control 73 also maytake a number of different forms. As indicated in FIG. 1, the screen 18is not operated or rotated until the differential water level D issensed by the system, whereupon operation of the screen commences. Ifoperation of the screen is successful to remove the debris which causedthe lowering of the water level on the pool side of the screen, thewater level on the pool side of the screen once again rises; and thesignals induced in the secondary winding 45 of the transformer 44increase. These singals become of sufficient magnitude to once againbuck or decrease the reverse bias applied to the grid of the tube 48 bythe winding 54 to the point where the current conducted through thewinding 61 of the relay 60 again becomes sufficient to withdraw thearmature 64 to the position shown in FIG. 1. The alarm 72 and thecontrol 73 then are rendered inoperative, and the original state ofconditions exists.

In the event that the water level on the right-hand or pool side of thescreen 18 continues to decrease past the level 69 where each of theelectrode pairs are immersed to the samedepth A illustrated in FIG. 2,the electrode pair 25 and 26 is immersed by a lesser amount than theelectrode pair 22 and 23. This causes the phase of the signals inducedin the secondary winding 45 to be reversed from the initial set ofconditions described above since the bridge becomes unbalanced in theopposite direction. Thus, the signals applied by the winding 45 to thetube 48 reverse to a series aiding phase relationship with the signalsapplied by the winding 54. This results in additional reduction of theconductivity of the triode 48 and insures that the relay 60 remainsdeenergized. If such a condition persists for a prolonged period oftime, it indicates that the operation of the screen 18 is insufficientto remove the clogging debris or perhaps the screen has failed tooperate. Thus, in addition to the circuit shown in FIG. 1, a timingcircuit or additional alarm circuit could be employed to shut down theequipment withdrawing water from the pool on the right-hand side of thescreen 18 if the relay 60 remains deenergized for more than apredetermined safe interval of time. Such additional control circuitry,however, would be initiated into operation by the release of the relay60 in the manner described above and could be implemented in a number ofconventional ways.

If it is desired to record the difi'erences in water level on oppositesides of the screen 18, an indicating recorder 75 may be connectedacross the grid-cathode biasing circuit for the triode 48. Connected inthis manner the recorder 75 may be used to record the variations inpotential of the bridge circuit as reflected by variations of the outputof the winding 45 combined with the constant output of the winding 54.

The screen 18 is grounded in the water 16 confined within the channelbut is not used as a common electrode or as a part of the bridge circuit29 since to do so could result in undersirable change in thedifferential setting by picking up other grounded objects as the poollevel rises and falls. For this reason, the closely spaced pairs ofelectrodes 22, 23 and 25, 26 are employed, with the spacing between theelectrodes of each of the pairs typically being of the order of 3 to 6inches. The electrode structure is most clearly shown in FIGS. 2, 3 and4, and the details of the electrode pair 22, 23 are shown in thecross-section and side views of FIGS. 3 and 4. It is to be understoodthat the electrode pair 25, 26 is identical to the electrode pair 22,23.

Each of the electrode pairs are mounted on a suitable mounting support80, which preferably is in the form of a U-shaped beam of steel or thelike having substantial mechanical strength since the lengths of theelectrode pairs for typical installations may be between and 50 feet ormore. The support member 80 is attached to a side wall of the waterchannel by a suitable number of spaced mounting brackets 81, one ofwhich is most clearly shown in FIGS. 3 and 4. The brackets 81 arefastened to the wall of the water channel with suitable fasteners, suchas conventional bolts and nuts One of the electrodes of each pair,illustrated as the electrode 22 or 25, is in the form of an elongatedbar of rectangular conductive material, preferably stainless steel,which is attached to the support 80 by any suitable means. At intervalsalong the length of the support 80, insulating spacers 85 are providedfor mounting the second electrode of each pair, illustrated as theelectrodes 23 and 26, parallel to the electrodes 22 and 25 respectively,as best shown in FIG. 4. Suitable clamps 86 are used to effect themounting of-the rods 23 and 26, on the spacers 85. The distance betweenadjacent ones of the insulating spacers 85 is not critical, with thenumber employed merely being sufiicient to provide sufficient mechanicalstrength and rigidity to the assembly. Typically the spacers 85 are usedat 4 to 6 foot intervals along the length of the support 80.

The electrodes 23 and 26 are made of the same material as the electrodes22 and 25 and are shown as stainless steel rods. Current between theelctrode 23 and the electrode 22 (and between the electrodes 26 and 25)is confined to a line formed between the nearest point of the rod 23with the center of the electrode. 22. Since both electrodes of theelectrode pairs 22, 23 and 25, 26 are made of the same material,deterioration of the electrodes by electrolysis is prevented. Inaddition, since the conductive paths between the electrode 23 and thesupport member 80 are greater than the nearest or shortest conductivepath between the center of the electrode 23 and the nearest point of theelectrode 23 (best seen in FIG. 3), erosion of the beam 80 byelectrolysis between it and the electrode 23 also is eliminated orsubstantially reduced, thereby prolonging the life of the assembly.

It should be noted that the circuit which has been described is notdependent upon the magnitude of the bridge output for supplying the biasto the tube 48 which causes the tube 48 conductivity to be reduced tothe point at which the relay 60 is released. The winding 54 of thestep-down transformer 55 supplies sufficient output with no signalinduced in the winding 45 to cause this to occur. Thus, absolutevariations in the water level from a very high level to an extremely lowlevel, which causes the resistance in both of the arms of the bridgeformed by the electrode pairs 22,. 23, and 25, 26 to rise significantly,do not prevent operation of the alarm circuit shown. Such absolutevariations in the water level on both sides of the screen 18substantially increase the impedance of the bridge thereby substantiallyreducing the output of the bridge applied across the primary winding 43.As described previously, however, the function of the transformer 44 is.such that with the bridge in balance, the winding 54 causes the tube 48to be biased to a point where the relay 60 is deenergized. Thus, it isnot necessary to provide additional circuitry to compensate for largevariations in the absolute or common changes of water level on bothsides of the screen.

I claim:

l. A system for sensing differences in the level of liquids at first andsecond spaced points including in combination:

first and second spaced conductive electrodes com prising a first pairof electrodes extending downwardly at the first of the spaced points forpartial immersion into liquid located at the first of said spacedpoints;

third and fourth spaced conductive electrodes comprising a second pairof electrodes extending downwardly at the second of the spaced pointsfor partial immersion into liquid located at the second of the spacedpoints, said second pair of electrodes extending into such liquid apredetermined distance greater than said first pair of electrodesextends with the liquid level at the first and second spaced pointsbeing the same;

resistance means connected between said first and 1 third electrodes atfirst and second junctions, re-

spectively;

means for interconnecting said second and fourth electrodes at a thirdjunction;

indicating means;

first means for connection with a primary source of electrical potentialfor impressing a difference of electrical potential between said firstand second junctions; bridge circuit output means coupled between saidthird junction and a point on said resistance means intermediate saidfirst and second junctions and I further coupled with said indicatingmeans for applying thereto a potential, the magnitude and phase of whichis indicative of differences in liquid level at said first and secondpositions; and

second means for connection with the primary source of electricalpotential coupled with said indicating means for applying apredetermined potential at a predetermined phase thereto.

2. The combination according to claim 1 wherein said first, second,third and fourth electrodes all are made of the same material.

3.'The combination according to claim 2 wherein said first, second,third and fourth electrodes are made of stainless steel.

4. The combination according to claim I wherein said first and secondmeans for connection with the primary source of electrical potentialcomprise first transformer means having at least one primary winding andfirst and second secondary windings corresponding to said first andsecond connection means, respectively, with the primary source ofelectrical potential being an alternating current potential supplied tosaid primary winding, said first secondary winding being connectedbetween said first and second junctions and said second secondarywinding being coupled in circuit with said indicating means; and saidbridge circuit output means comprises a second transformer having aprimary winding coupled between said third junction and said point onsaid resistance means and having a secondary winding coupled with saidindicating means.

5. The combination according to claim 4 wherein the coupling of saidsecond secondary winding of said first tranformer means with saidindicating means is in series circuit through said secondary winding ofsaid second transformer.

6. The combination according to claim 5 wherein said first transformermeans comprises step-down transformer means and said second transformercomprises a step-up transformer.

7. The combination according to claim 6 wherein the second secondarywinding of said first transformer means and the secondary winding ofsaid second transformer are connected together in series circuit withsaid indicating means, the imbalance of said bridge circuit for the sameliquid level at the first and second spaced points causing a signal tobe produced in the secondary winding of said second transformer which isin phase opposition to the signal from the second secondary winding ofsaid first transformer means to cause a first output to be obtained fromsaid indicating means, balancing of said bridge removing such phaseopposition signal with the signal from the second secondary winding ofsaid first transformer means in the absence of such phase oppositionsignal being of sufficient magnitude to cause a second output to beobtained from said indicating means.

8. A liquid level sensing system for sensing differences in the level ofa liquid on opposite sides of a screen disposed in a stream channelincluding in combination:

an alternating current power supply circuit; a relay with a winding andhaving a normal condition of operation and an alarm condition ofoperation;

an electronic control device having an output circuit connected inseries with said relay winding to said power supply circuit and having acontrol electrode; first and second spaced conductive electrodescomprising a first pair of electrodes extending downwardly on one sideof the screen for partial immersion into liquid located on said one sideof the screen; third and fourth spaced conductive electrodes comprisinga second pair of electrodes extending downwardly at the other side ofthe screen for partial immersion into liquid located at said other sideof the screen, the ends of said third and fourth electrodes extendingdownwardly beyond the ends of said first and second electrodes apredetermined distance;

resistance means connected between said first and third electrodes atfirst and second junctions, respectively;

means for interconnecting said second and fourth electrodes at a thirdjunction to complete a bridge circuit comprising said resistance meansand said first and second pairs of electrodes;

supply transformer means having at least one primary winding and firstand second secondary windings, the primary winding of which is coupledwith said alternating current power supply circuit, the first secondarywinding of which is connected across said first and second junctions;

a bridge output tranformer having a primary winding and a secondarywinding, with the primary winding connected between said third junctionand an intermediate point on said resistance means;

means for connecting the secondary winding of said bridge transformer inseries circuit with the second secondary winding of said supplytransformer means and the control electrode of said electronic controldevice, said electronic control device causing the relay to be in itsnormal condition of operation with the secondary winding of said bridgetransformer supplying AC signals in phase opposition to the signalsapplied to the electronic control device from the second secondarywinding as a result of a bridge circuit imbalance caused by thedifferences in deth of immersion of said first and second pairs ofelectrodes for liquid of the same level on opposite sides of the screen,the signals from the second secondary winding being sufficient to causesaid electronic control device to operate said relay to said alarmcondition of operation in the absence of phase opposition signals fromthe secondary winding of said bridge transformer; and

means coupled with said relay circuit for responding to said alarmcondition of operation thereof.

9. The combination according to claim 8 wherein said first transformermeans comprises first and second transformers having first and secondprimary windings coupled with said alternating current power supply circuit, said first and second transformers being stepdown transformershaving the first and second secondary windings, respectively, and saidbridge output transformer comprising a step-up transformer.

10. The combination according to claim 8 wherein said electronic controldevice comrises a triode tube, with an anode, cathode and grid, havingthe anode and cathode connected in series circuit with said relaywinding to said power supply circuit, with the second secondary windingand the secondary winding of said bridge output transformer beingconnected in series circuit across the grid and cathode of said triode,the signals induced in said second secondary winding being sufficientto'cause said triode to be rendered nonconductive in the absence ofsignals of opposite phase from the secondary winding of said bridgeoutput transformer.

1. A system for sensing differences in the level of liquids at first andsecond spaced points including in combination: first and second spacedconductive electrodes comprising a first pair of electrodes extendingdownwardly at the first of the spaced points for partial immersion intoliquid located at the first of said spaced points; third and fourthspaced conductive electrodes comprising a second pair of electrodesextending downwardly at the second of the spaced points for partialimmersion into liquid located at the second of the spaced points, saidsecond pair of electrodes extending into such liquid a predetermineddistance greater than said first pair of electrodes extends with theliquid level at the first and second spaced points being the same;resistance means connected between said first and third electrodes atfirst and second junctions, respectively; means for interconnecting saidsecond and fourth electrodes at a third junction; indicating means;first means for connection with a primary source of electrical potentialfor impressing a difference of electrical potential between said firstand second junctions; bridge circuit output means coupled between saidthird junction and a point on said resistance means intermediate saidfirst and second junctions and further coupled with said indicatingmeans for applying thereto a potential, the magnitude and phase of whichis indicative of differences in liquid level at said first and secondpositions; and second means for connection with the primary source ofelectrical potential coupled with said indicating means for applying apredetermined potential at a predetermined phase thereto.
 2. Thecombination according to claim 1 wherein said first, second, third andfourth electrodes all are made of the same material.
 3. The combinationaccording to claim 2 wherein said first, second, third and fourthelectrodes are made of stainless steel.
 4. The combination according toclaim 1 wherein said first and second means for connection with theprimary source of electrical potential comprise first transformer meanshaving at least one primary winding and first and second secondarywindings corresponding to said first and second connection means,respectively, with the primary source of electrical potential being analternating current potential supplied to said primary Winding, saidfirst secondary winding being connected between said first and secondjunctions and said second secondary winding being coupled in circuitwith said indicating means; and said bridge circuit output meanscomprises a second transformer having a primary winding coupled betweensaid third junction and said point on said resistance means and having asecondary winding coupled with said indicating means.
 5. The combinationaccording to claim 4 wherein the coupling of said second secondarywinding of said first tranformer means with said indicating means is inseries circuit through said secondary winding of said secondtransformer.
 6. The combination according to claim 5 wherein said firsttransformer means comprises step-down transformer means and said secondtransformer comprises a step-up transformer.
 7. The combinationaccording to claim 6 wherein the second secondary winding of said firsttransformer means and the secondary winding of said second transformerare connected together in series circuit with said indicating means, theimbalance of said bridge circuit for the same liquid level at the firstand second spaced points causing a signal to be produced in thesecondary winding of said second transformer which is in phaseopposition to the signal from the second secondary winding of said firsttransformer means to cause a first output to be obtained from saidindicating means, balancing of said bridge removing such phaseopposition signal with the signal from the second secondary winding ofsaid first transformer means in the absence of such phase oppositionsignal being of sufficient magnitude to cause a second output to beobtained from said indicating means.
 8. A liquid level sensing systemfor sensing differences in the level of a liquid on opposite sides of ascreen disposed in a stream channel including in combination: analternating current power supply circuit; a relay with a winding andhaving a normal condition of operation and an alarm condition ofoperation; an electronic control device having an output circuitconnected in series with said relay winding to said power supply circuitand having a control electrode; first and second spaced conductiveelectrodes comprising a first pair of electrodes extending downwardly onone side of the screen for partial immersion into liquid located on saidone side of the screen; third and fourth spaced conductive electrodescomprising a second pair of electrodes extending downwardly at the otherside of the screen for partial immersion into liquid located at saidother side of the screen, the ends of said third and fourth electrodesextending downwardly beyond the ends of said first and second electrodesa predetermined distance; resistance means connected between said firstand third electrodes at first and second junctions, respectively; meansfor interconnecting said second and fourth electrodes at a thirdjunction to complete a bridge circuit comprising said resistance meansand said first and second pairs of electrodes; supply transformer meanshaving at least one primary winding and first and second secondarywindings, the primary winding of which is coupled with said alternatingcurrent power supply circuit, the first secondary winding of which isconnected across said first and second junctions; a bridge outputtranformer having a primary winding and a secondary winding, with theprimary winding connected between said third junction and anintermediate point on said resistance means; means for connecting thesecondary winding of said bridge transformer in series circuit with thesecond secondary winding of said supply transformer means and thecontrol electrode of said electronic control device, said electroniccontrol device causing the relay to be in its normal condition ofoperation with the secondary winding of said bridge transformersupplying AC signals in phase opposition to the signals applied to theelectronic control device from The second secondary winding as a resultof a bridge circuit imbalance caused by the differences in deth ofimmersion of said first and second pairs of electrodes for liquid of thesame level on opposite sides of the screen, the signals from the secondsecondary winding being sufficient to cause said electronic controldevice to operate said relay to said alarm condition of operation in theabsence of phase opposition signals from the secondary winding of saidbridge transformer; and means coupled with said relay circuit forresponding to said alarm condition of operation thereof.
 9. Thecombination according to claim 8 wherein said first transformer meanscomprises first and second transformers having first and second primarywindings coupled with said alternating current power supply circuit,said first and second transformers being step-down transformers havingthe first and second secondary windings, respectively, and said bridgeoutput transformer comprising a step-up transformer.
 10. The combinationaccording to claim 8 wherein said electronic control device comrises atriode tube, with an anode, cathode and grid, having the anode andcathode connected in series circuit with said relay winding to saidpower supply circuit, with the second secondary winding and thesecondary winding of said bridge output transformer being connected inseries circuit across the grid and cathode of said triode, the signalsinduced in said second secondary winding being sufficient to cause saidtriode to be rendered nonconductive in the absence of signals ofopposite phase from the secondary winding of said bridge outputtransformer.